Those skilled in the art will appreciate that when a digital signal is received for processing it is necessary to establish synchronization between the receiver and the incoming data stream. Historically, digital communication systems that transmitted stored data, such as computer data, experienced little difficulty in adding a synchronization signal since the data could be stopped and re-started at any time, without loss of continuity as viewed from the receiving computer terminal. However, this technique does not readily apply to interleaving a synchronization signal into a continuous voice message since it may lead to an intolerably interrupted received message.
Accordingly, the self-synchronous digital communication system, wherein the data is of a non-framed, serial nature, became widely used. In this system, a clock which can be recovered from the incoming data bits, is all that is required to synchronize the system. There are cases, however, where additional synchronization information is needed. For example, if the serial data is encrypted, the state of the transmitter encryption device must be sent to the receiver decryption device so that the data can be decrypted. It is possible to have the encryption process also be self-synchronous, requiring only the encrypted data stream to synchronize the receiver, but this introduces a detriment to the system in the form of range reduction. This is brought on with self-synchronous encryption by an error made at the receiver being multiplied by the decryption process to add a burst of errors to the decrypted data By sending the state of the transmitter encryption device as synchronization information periodically within the encrypted digital signal, it is possible for the receiver to extract the synchronization information without the penalty of error multiplication. This allows encryption to be used without the penalty of range reduction present with the self-synchronous encryption systems. However, no convenient method exists for interleaving synchronization information into a digital voice signal. Therefore, a need exists for a convenient method to allow additional synchronizing information to be added to continuous digital voice information.